In recent years, solid-state imaging devices have come to be employed widely in the imaging portion of, for example, composite video cameras and digital still cameras. Of these, interline-transfer type CCD solid-state imaging devices (hereinafter, referred to as IT-CCDs) are particularly popular because of their low noise properties.
FIG. 8 is a diagram that schematically shows the configuration of an ordinary IT-CCD. In FIG. 8, reference numeral 1 denotes photodiodes having a photoelectric conversion function, 2 denotes vertical transfer portions that have a buried channel structure and that are for transferring signal charges in the vertical direction, 3 denotes vertical transfer gates that control vertical transfer, 4 denotes a horizontal transfer portion for transferring signal charges in the horizontal direction, and 5 denotes an output portion.
FIG. 9 is a diagram illustrating a unit pixel P, which includes a photodiode 1, a vertical transfer portion 2, and a vertical transfer gate 3 of FIG. 8. FIG. 10 schematically shows a cross-section taken along the line A–A′ in FIG. 9. In FIG. 10, the photodiode 1 and the vertical transfer portion 2 are formed within a silicon substrate 11. The vertical transfer gate 3 is formed on the silicon substrate 11. Reference numeral 6 denotes a light-blocking film that has been provided such that incident light is kept from being incident on regions other than the photodiode 1, such as on the vertical transfer portion 2. 8a and 8b are a first and a second dielectric film, respectively, having SiO2 as a main component, and 10 is a protective film. An organic dielectric film 12 is formed on the protective film 10 and planarized. A lens 7 made of an organic film is formed on the organic dielectric film 12, and focuses incident light into the photodiode 1. The dielectric film 12 functions both as a planarizer and as a color filter.
FIGS. 11A and 11B show the process steps in producing the above conventional solid-state imaging device. FIG. 11A shows a cross-section at a state where the light-blocking film 6, the second dielectric film 8b, and then the protective film 10 have been formed. It should be noted that after the second dielectric film 8b is formed, it is subjected to a thermal flow process to provide it in the shape illustrated here. After the protective film 10 is formed, as shown in FIG. 11B, the organic dielectric film 12 and then the lens 7 are formed.
However, the solid-state imaging device of the above structure has the problem that it cannot effectively utilize the incident light when focusing by the lens 7 is not sufficient. That is, when light is perpendicularly incident on the solid-state imaging device, it is effectively focused by the lens 7 and usefully incident on the photodiode 1, but when the angle of incidence has deviated from the perpendicular direction, the incident light is not focused onto the photodiode 1 and is diffusely reflected by the surface of the light-blocking film 6, and this did not allow the incident light to be effectively utilized.
In particular, as cameras have become more compact, the miniaturization of the unit pixels of solid-state imaging devices and the shortening of the exit pupil length of the lens used in cameras have become remarkable, and thus the problem mentioned above has become even more pronounced. For example, although more compact unit pixels have led to a shrinking of the photodiode aperture width W, which is the aperture of the light-blocking film 6, the film thickness of the vertical transfer gates 3 cannot be provided thin proportional to the extent to which the aperture width is reduced. This has resulted in a structure having a pit shape with a narrow aperture, making focusing of the incident light difficult. Further, shorter exit pupil distances in the camera lens are one cause for the increase in the ratio of light incident on the solid-state imaging device whose angle has deviated from the perpendicular direction, and this, too, makes it difficult to achieve effective focusing of incident light onto the photodiode 1.
In response to the above problems, Japanese Patent No. 2869280 discloses a structure for increasing the sensitivity, resolution, and image quality by providing a low refraction region layer in the lateral wall of the light path formation portion positioned above the photoelectric conversion portion, so as to cause light that is incident into the lateral area of the transfer electrode or light that is diffused to adjacent pixels to be incident on the photoelectric conversion portion. Japanese Patent No. 2869280 discloses a method for forming a gas layer as the low refraction region layer by applying a soluble resin, covering that resin with another resin, and then dissolving the soluble resin to form the gas layer (paragraphs 0008 and 0014; see FIG. 1).
The structure of the low refraction region layer disclosed in Japanese Patent No. 2869280, however, is not sufficient for focusing the light that is incident on the area above the photodiode area onto the photodiode over a wide range.
In other words, that light that is incident on an intermediate region between the photodiode 1 and other surrounding photodiodes is incident on the light-blocking film 6 at an angle close to a right angle, and thus reflection occurs at the surface of the light-blocking film 6 and it was not possible to focus the light that is incident on this region onto the photodiode 1.
Also, with the manufacturing method disclosed in Japanese Patent No. 2869280, it was difficult to uniformly apply a thin soluble resin onto the surface of a solid-state imaging device that has severe unevenness, because liquid pools are formed in the recessed portions, entire recessed portions are buried with the resin, or bubbles without resin are formed in some of the recessed portions. Thus it was not easy to obtain a low refraction region layer with uniform properties.